1. Field of the Invention
The present invention relates to a non-aqueous secondary battery and a negative electrode for the non-aqueous secondary battery. More precisely, it relates to a non-aqueous secondary battery having a negative electrode of a carbon material capable of absorption-desorption or intercalation-deintercalation of lithium into or from itself, and also to the negative electrode for the battery.
2. Related Art
With the recent advances in small-sized and energy-saving electronic appliances, secondary batteries using alkali metals such as lithium, etc. have become prevalent.
When a simple substance of lithium metal is used as the negative electrode of a battery, dendrites (dendritic crystals) are formed on the surface of the metal due to the repetition of charging-discharging cycles of the negative electrode (deposition-dissolution cycles of lithium metal). The growth of the dendrites causes a problem in that the grown dendrites penetrate through the separator and are brought into contact with the positive electrode to induce the short-circuit in a battery.
When a lithium alloy is used, in place of lithium metal, as the negative electrode of a secondary battery, the formation of dendrites is retarded with the result that the characteristic of charging-discharging cycles of the battery is improved, as compared with the case where the simple substance of lithium metal is used. However, even though such a lithium alloy is used, the formation of dendrites is not completely inhibited, but still has the possibility of the short-circuit in a battery. In addition, the use of such an alloyed negative electrode results in the increase in the weight of the battery, which therefore detracts from the light weight advantage of lithium-containing secondary batteries.
Recently, matrix materials such as carbon materials, electroconductive polymers, etc. capable of absorption-desorption of lithium ions into or from themselves have been developed and used as the negative electrodes of batteries, in place of lithium metal or alloys. In principle, these materials are free from the problem of the formation of dendrites, which, as mentioned above, is inevitable in the negative electrodes comprising lithium metal or alloys,. Therefore, the use of these materials as the negative electrodes of batteries has resulted in the noticeable decrease in the problem of short circuiting of the batteries. In particular, carbon materials are preferred to the other matrix materials since their potential for absorption-desorption of lithium into or from them is nearer to the potential of lithium for its deposition-dissolution than the others. Above all, a graphite material can take theoretically one lithium atom per 6 carbon atoms in its crystal lattices. Therefore, such a graphite material has a high capacity per the unit weight of the carbon of itself (372 mAh/g; see Phys. Rev. B., Vol. 42, 6242 (1990)). In addition, a graphite material is chemically stable and therefore significantly contributes to the cycle stability of batteries having it as the negative electrode.
However, since there occur unfavorable side reactions such as the decomposition of the solvent used at the initial charging of a carbon material, the capacity loss of the carbon material is inevitable. Therefore, carbon materials are disadvantageous in that their initial charging efficiency is low since the electric capacity necessary for charging them is higher than that necessary for discharging them (see J. Electrochem. Soc., Vol. 137, 2009 (1990)). For these reasons, it is important to prevent the capacity loss of carbon materials if the materials are used as the negative electrodes of high-capacity secondary batteries.
In general, various functional groups exist on the surface of a carbon material and such functional groups act as active points in various reactions. Therefore, these functional groups act as the active points also in lithium secondary batteries having a carbon material as the negative electrode, thereby causing the irreversible capacity of the batteries.
In Japanese Patent Laid-Open No. 5-114421, there is proposed the use of a carbon material that has been chemically treated to esterify the functional groups on its surface, as the negative electrode of a battery, in order to solve the above-mentioned problem. The proposed means is effective for a carbon material having a relatively large specific surface area but is not satisfactory for a graphite material having a small specific surface area.
The proposed means of Japanese Patent Laid-open No. 5-114421 is to merely esterify the functional groups on the surface of a carbon material but is not to basically remove the active points from its surface. In addition, since it uses an organic solvent (e.g., ethyl alcohol) for the chemical treatment, the carbon material treated must be dried so as to remove the organic solvent therefrom.
In Japanese Patent Laid-Open Nos. 5-135802 and 5-299074, it is proposed to previously treat a graphite material by a wet process and thereafter calcine and dry it in an inert gas or in a vacuum thereby compensating the capacity loss, such as that mentioned hereinabove, of the material. The proposed method, as comprising the wet treatment, needs water or an organic solvent. If the graphite material treated bv the proposed method is used as the negative electrode in a batterv along with highly-reactive lithium as the active material, the water or solvent used for treating the graphite material must be completely removed.
In Japanese Patent Laid-Open No. 5-28996, it is proposed to treat a carbon material of natural graphite under heat in an iner t gas in order to reduce the capacity loss, suc h as that mentioned hereihnabove, of the material. However, this method is not effective for carbon materials and graphite materials other than natural graphite.
As mentioned hereinabove, there exist various oxygen-containing functional groups such as hydroxyl group (OH), carboxyl group (COOH) and carbonyl group (CO) and various oxygen-containing bonds such as ester bond (COO) on the surfaces of carbon materials. Since these groups and bonds are highly reactive winh lithium, these react with lithium, at the initial chariing of batteries containing a carbon material and lithium, to yield an irreversible capacity. Therefore, in producing batteries, it is necessary to previouslv incorporate into batteries an active material for the nositive electrode containing the necessary amount of lithium corresponding to the irreversible capacity. However, such incorparation, is problematic in that the capacity density of t h e batteries to be produced is lowered by the amount of lithium in corporated.
The oxygen-containing functional groups existing on the surface of the carbon material used as the negative electrode of a battery react with lithium stored in the carbon material while the ealtero es charged, thereby causing the self-discharging of the battery.
In addition, the oxygen-containing functional groups react with the electrolytic solution in the battery to deplete the electrolytic solution or to generate gas in the battery to thereby increase the internal pressure of the battery.